Add @syntax-background-color guard

In case the variable doesn't exist, it falls back to the core variable https://github.com/atom/atom/blob/master/static/variables/syntax-variables.less#L9

Note: The color in the guard actually doesn't matter. For some reason the one in atom/static is used.

Issue atom/one-light-ui#16

Author: simurai

styles/ui-variables.less

@@ -3,6 +3,7 @@
 ** ---------------------------------------------- */
 
 @import "syntax-variables";
+.ui-syntax-color() { @syntax-background-color: hsl(220,24%,20%); } .ui-syntax-color(); // fallback color
 @ui-syntax-color: @syntax-background-color;
 
 // Color guards -----------------